Vascular wound repair is controlled by a complex interaction between local vascular cells and circulating immune and non-immune cells. Recently, the classic dogma of a predominantly local response to injury has been challenged by the identification of circulating vascular progenitor cells. The role of these vascular progenitor cells in vascular regeneration and their interaction with local vascular cells and infiltrating immune cells is poorly understood. My laboratory has undertaken a number of approaches to understand how these cells contribute to the overall vascular injury response and how they might ultimately be therapeutically manipulated.[unreadable] We have used a variety of mouse models to better understand what regulates the interplay of local and circulating cells following injury. One of our initial approaches was to investigate whether p21Cip1 expression modulates the interactions between local vascular cells and circulating immune cells. We found that mechanical injury of the femoral artery in p21-/- mice accelerated vascular smooth muscle cell proliferation and increased infiltration of circulating immune cells compared to p21+/+ mice. Bone marrow transplantations between p21-/- and p21+/+ mice indicated an important role of p21Cip1 expression in resident vascular cells. Deletion of p21Cip1 increased apoptosis and SDF-1 level in the vasculature during vascular wound repair. Similarly, antagonism of the SDF-1 receptor, CXCR4, in vivo decreased infiltration of monocytes and neointima formation in p21-/- mice. [unreadable] We have also embarked on a number of studies to determine the origin and fate of progenitor cells involved in vascular remodeling. We are using a Cre-Lox-based lineage tracking system to genetically track the fate of vascular smooth muscle and endothelial cells in vivo. We are also utilizing a Tie2Cre/R26RLacZ cell fate mapping system in a murine arterial-vein graft model to investigate the role of endothelial-derived cells in neointima formation. In addition, because long-term label-retaining cell (LRC) approaches have been previously successful in identifying stem cell populations within their niche, we have employed this approach to the vessel wall. Preliminary results suggest that a small population of LRCs exist in close proximity to blood vessels. We believe these cells may represent resident somatic stem/progenitor cells and, as such, we are currently determining the surface marker phenotypes of these newly discovered and potentially unique cells. [unreadable] We have also explored the utility of embryonic stem cells (ESCs) as an in vitro model to study the differentiation of vascular cells and cardiomyocytes. Following developmental programs for endothelial cell differentiation, we have successfully differentiated murine ESCs into hematovascular and cardio-mesodermal precursor cells and their progenies. We have identified unique surface markers to further distinguish the cardiac and vascular progenitor cells, and are currently testing these purified cell types in murine models of cardiovascular disease. [unreadable] The identification of local and circulating vascular progenitor cells also provides the rationale for therapeutic application of stem/progenitor cells for cardiovascular disease. Multipotent adult progenitor cells (MAPCs) were one of the first described adult pluripotent progenitor cells purified from the bone marrow of adult humans, mice and rats. We isolated MAPC-derived progenitor cells (MDPCs) from rat bone marrow and transplanted them into the ischemic muscle of a rat hind limb model. After four weeks, MDPCs were retained in the perivascular area of the ischemic leg. Although MDPCs did not differentiate into vascular cells, perfusion was increased through the paracrine mechanism and by local modulation of the inflammatory response.[unreadable] In summary, we are using a wide range of approaches including studying targeted mouse genetic models, developing cell fate mapping and cell lineage depletion strategies, and analyzing murine embryonic stem cells (mESCs), all in an effort to explore the complex cellular interactions and regulatory pathways that occur during vascular regeneration and remodeling. Our main goal is to increase our knowledge in this area of vascular biology and eventually translate this understanding to the clinical setting.